1. Field of the Invention
The present invention relates to a pushbutton mechanism for keyboards and particularly to a pushbutton mechanism that has a bridge interposed between a key cap and a base plate to provide a steady relative turning and confine the pushbutton stroke.
2. Brief Description of the Related Art
The conventional pushbutton mechanism for thin keyboards mainly includes a bridge (or called scissors-type moving mechanism), a circuit board, an actuation means and a base plate (some also have the bridge mounting onto a bridge board, and have the circuit board interposed between the bridge board and the base plate). The techniques of the bridge mechanism can be divided into two types, each has its benefits and drawbacks as discussed below:
1. U.S. Pat. Nos. 5,278,374, 5,555,971, 5,964,341 and 6,428,223 propose a bridge that has an inner frame and an outer frame with corresponding stub shafts and pivot portions on the side brackets. The inner frame and outer frame are coupled on an axle which can reduce rotational friction to make turning smoother. But the inner and outer frames are difficult to fabricate and couple together. To control turning of the bridge to achieve a desired pushbutton stroke, extending lugs have to be provided for harness. This technique was disclosed in R.O.C. patent publication No. 511772.
2. U.S. Pat. No. 5,630,501 discloses a technique which has an inner frame and outer frame with mating bosses and recesses coupled in a straddle manner. The inner and outer frames can be fabricated and coupled together, and the bosses and recesses are corresponding to each other and can control the pushbutton stroke. An improvement is disclosed in R.O.C. patent publication No. 539177. However, the straddle coupling does not provide a desired positioning harness. A greater turning friction occurs and the frames could be separated.
3. The aforesaid conventional techniques have a common problem, namely when the inner and outer frames of the bridge are coupled together, they have to maintain a constant allowance in normal conditions during turning relative to each other to avoid contact friction between them so that wearing or fracturing can be prevented. But the present techniques of the bridge do not provide a harness mechanism for the inner and outer frames. The inner and outer frames could have transverse movements and result in damages.